Fuel cell technology shows great promise as an alternative energy source for numerous applications. Several types of fuel cells have been constructed, including polymer electrolyte membrane fuel cells, direct methanol fuel cells, alkaline fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. For a comparison of several fuel cell technologies, see Los Alamos National Laboratory monograph LA-UR-99-3231 entitled Fuel Cells: Green Power by Sharon Thomas and Marcia Zalbowitz.
FIG. 1 represents an example of a fuel cell 100, including a high surface area anode 110 including an anode catalyst 112, a high surface area cathode 120 including a cathode catalyst 122, and an electrolyte 130 between the anode and the cathode. The electrolyte may be a liquid electrolyte; it may be a solid electrolyte, such as a polymer electrolyte membrane (PEM); or it may be a liquid electrolyte contained within a host material, such as the electrolyte in a phosphoric acid fuel cell (PAFC).
In operation of the fuel cell 100, fuel in the gas and/or liquid phase is brought over the anode 110 where it is oxidized at the anode catalyst 112 to produce protons and electrons in the case of hydrogen fuel, or protons, electrons, and carbon dioxide in the case of an organic fuel. The electrons flow through an external circuit 140 to the cathode 120 where air, oxygen, or an aqueous oxidant (e.g., peroxide) is being fed. Protons produced at the anode 110 travel through electrolyte 130 to cathode 120, where oxygen is reduced in the presence of protons and electrons at cathode catalyst 122, producing water in the liquid and/or vapor state, depending on the operating temperature and conditions of the fuel cell.
Hydrogen and methanol have emerged as important fuels for fuel cells, particularly in mobile power (low energy) and transportation applications. The electrochemical half reactions for a hydrogen fuel cell are listed below.
Anode:2H2→4 H+ + 4 e−Cathode:O2 + 4 H+ + 4 e−→2 H2OCell Reaction:2 H2 + O2→2 H2OTo avoid storage and transportation of hydrogen gas, the hydrogen can be produced by reformation of conventional hydrocarbon fuels. In contrast, direct liquid fuel cells (DLFCs) utilize liquid fuel directly, and do not require a preliminary reformation step of the fuel. As an example, the electrochemical half reactions for a Direct Methanol Fuel Cell (DMFC) are listed below.
Anode:CH3OH + H2O→CO2 + 6 H+ + 6 e−Cathode:1.5 O2 + 6 H+ + 6 e−→3 H2OCell Reaction:CH3OH + 1.5 O2→CO2 + 2 H2O
A key component in conventional fuel cells is a semi-permeable membrane, such as a solid polymer electrolyte membrane (PEM), that physically and electrically isolates the anode and cathode regions while conducting protons (H+) through the membrane to complete the cell reaction. Typically, PEMs have finite life cycles due to their inherent chemical and thermal instabilities. Moreover, such membranes typically exhibit relatively poor mechanical properties at high temperatures and pressures, which can seriously limit their range of use.
In contrast, a laminar flow fuel cell (LFFC) can operate without a PEM between the anode and cathode. An LFFC uses the laminar flow properties of a microfluidic liquid stream to deliver a reagent to one or both electrodes of a fuel cell. In one example of an LFFC, fuel and oxidant streams flow through a microfluidic channel in laminar flow, such that fluid mixing and fuel crossover is minimized. In this example, an induced dynamic conducting interface (IDCI) is present between the two streams, replacing the PEM of a conventional fuel cell. The IDCI can maintain concentration gradients over considerable flow distances and residence times, depending on the dissolved species and the dimensions of the flow channel. IDCI-based LFFC systems are described, for example, in U.S. Pat. No. 6,713,206 to Markoski et al., in U.S. Pat. No. 7,252,898 to Markoski et al., and in U.S. Patent Application Publication 2006/0088744 to Markoski et al.
An LFFC can be operated with a single flowing electrolyte. The use of one flowing electrolyte in a microfluidic channel, instead of two flowing electrolytes, may provide additional advantages, such as increased simplicity of the fuel cell and smaller physical dimensions for the cell. Single flowing electrolyte based LFFC systems are described, for example, in U.S. patent application Ser. No. 12/061,349, filed Apr. 2, 2008, entitled “Microfluidic Fuel Cells”. IDCI-based LFFC systems and LFFC systems using a single flowing electrolyte are each examples of flowing electrolyte fuel cells.
One challenge faced in developing fuel cells is to reduce their physical dimensions without sacrificing their electrochemical performance. As the dimensions of a flowing electrolyte fuel cell are reduced, it becomes increasingly difficult to maintain uniform distribution of reagents and temperature throughout the cell. It would be desirable to provide a fuel cell that has the advantages and electrochemical performance of a flowing electrolyte fuel cell, but that has smaller physical dimensions. It would also be desirable to ensure that reagents and operating temperature are uniformly distributed throughout the cell.